When you install Snow Leopard or migrate to Snow Leopard, known-incompatible software is moved to a folder named Incompatible Software on your hard drive.

Snow Leopard also prevents known-incompatible software from opening. If you see an "Incompatible software" message, contact the software's vendor or visit their website for a later, compatible version.

Restricted software includes Parallels, EyeTV, and even a few of Apple's own applications, like Aperture and Keynote (albeit old versions of them). Check the previously mentioned application compatibility wiki for a more robust user-generated list of compatible/incompatible apps, and—if you've got a must-have that's not Snow Leopard-ready—let's hear about it in the comments.